Communication systems are widely used to provide services, such as voice, video, data, messaging, broadcast, etc. These systems can be multiple-access and support a plurality of users who share the available system resources. Code division multiple access (CDMA) systems, such as the UMTS system and its evolutions, HSDPA and HSUPA, or frequency division systems (FDMA and OFDMA) are examples of these systems.
A typical communication system comprises a plurality of cells, each one divided into a plurality of sectors. Each cell contains a base station subsystem, BSS in GSM systems, or a radio network subsystem, RNS in UMTS systems, able to provide communication services to each user terminal (UE) located in that cell. Each base station subsystem BSS/RNS comprises a plurality of base transceiver stations, BTS in GSM systems and NodeB in UMTS systems, each able to communicate wirelessly with the user terminals UE located in the sectors of the cell served by the BSS. Many communication systems, such as the GSM or UMTS systems and their evolutions, also comprise a radio network controller, BSC in GSM systems and RNC in UMTS systems, in signal communication with each BSS/RNS and a plurality of user terminals, each one in signal communication with one or more BTS/NodeB.
The connection between the user terminals UE and the BTSes is defined by a downlink leg, from the BTS to the user terminals UE, and an uplink leg, from the user terminals UE to the BTS.
It should be pointed out that the user terminals UE can transmit simultaneously in uplink to the BTS and thus cause general interference in the transmission from other terminals UE to the BTS. In particular, the quality of a signal received by a BTS from a terminal UE depends on various factors, including the power transmitted from the terminal UE, losses on the path from the terminal UE to the BTS, interference generated by other terminals UE, phase shifts introduced on the signal itself due to multiple paths created following signal reflection and or refraction when obstacles are encountered, etc.
In consequence, total interference at the BTS increases as the transmitted power from the terminals UE increases and as the number of terminals UE increases.
Furthermore, as interference increases, the system instructs the single terminal UE to increase the power of the transmitted signal in order to receive said signal with a minimum quality that is sufficient to decode the communication correctly.
On the other hand, the terminals UE cannot increase transmitted power beyond their maximum nominal power and the communication system, in order to preserve the possibility for all terminals UE to communicate with the BTS, cannot tolerate an increase in interference beyond a predefined limit. Thus, in the presence of a plurality of terminals UE, the capacity of the system is limited by the interference on the uplink leg.
A measure of the interference on the uplink leg is provided by the Rise-over-Thermal (RoT) parameter, defined as the ratio between total noise and the interference produced by the terminals UE with the thermal noise in a cell.
The RoT parameter therefore represents a fundamental measurement for controlling the load on the uplink.
The RNS must therefore guarantee an overall interference received by the BTS that does not compromise the coverage of the cell itself, setting an admissible maximum interference limit, namely a RoT threshold.
The 3GPP UMTS standard contemplates that the network access controller RNC communicates the maximum admissible cell load to the NodeB. This communication takes place via the NBAP Physical Shared Channel Reconfiguration procedure. In detail, the RNC indicates the total maximum power in reception (Maximum Target Received Total Wide Band Power) and, optionally, the reference background noise (Reference Received Total Wide Band Power) in the NBAP Physical Shared Channel Reconfiguration Request message.
The RoT parameter represents the ratio between these two values and, as known in the literature, the capacity of the uplink leg has a logarithmic relation with the RoT value, so that the larger the allowed RoT value, the greater is the capacity of the cell and the maximum throughput serviceable from the NodeB.
On the other hand, it is necessary to note that an increase in the RoT threshold value causes a contraction of the uplink leg's coverage. Initially, the contraction in coverage, due to the increase in interference from the mobile terminals, is compensated by the rise in power of the mobile terminals through the known “fast power control” functionality.
When the power being used reaches the maximum nominal transmission power for the mobile terminal, said compensation is no longer possible and the need arises to perform a handover to other systems, if present. In the absence of these actions, there is the risk of impairing the quality of the communication or even of not being able to continue the communication. This problem typically occurs on the mobile terminals furthest away from the NodeB antennas.
From the above explanation, it follows that to keep the load on the uplink leg below a preset level and avoid system instability, it is fundamental to estimate the RoT threshold value in a precise manner.
WO 2004/114715 describes a method and an apparatus for the dynamic adjustment of the RoT threshold in a wireless communications system. In particular, the RoT threshold is dynamically increased or decreased upon detection, by a RoT threshold processor of a radio base station, of the outage of at least one terminal UE. The RoT threshold is initially set to a preset minimum value ROT_MIN and the RoT threshold processor checks for the presence of outage on one of the terminals UE registered with the radio base station, for example, the terminal with the lowest transmitted data flow. If an outage is detected, the ROT threshold is reduced by a certain value (ROT_DOWNSTEP) and the radio base station sets a bit RA to 1 to signal to all the terminals that communicate with the radio base station to reduce the flow of transmitted data. Successively, the processor checks if an outage occurs on another terminal and, if negative, increases the ROT threshold by a preset value (ROT_UPSTEP) that, for example, could be less that the decrease ROT_DOWNSTEP so as to maintain a low probability of outage.
The above-described technique, although allowing the RoT threshold value to be changed dynamically and optimized, has some drawbacks.
In fact, it should be noted that the value of the RoT threshold is decreased whenever an outage occurs on a terminal, or rather when the connection between the mobile radio station and the terminal is lost. Basically, the method described in WO 2004/114715 reacts to the outage of a terminal, but is not able to prevent this outage.
WO 2005/112485 describes a method to facilitate the uplink transfer of data from a user terminal. The method contemplates periodically determining the RoT level, periodically transmitting a RoT level indicator from a NodeB to a user terminal over a first control channel, periodically determining a mean aggregate network load value and periodically transmitting an indicator of this mean aggregate load value from the NodeB to the user terminal over a second control channel. The NodeB measures the instantaneous RoT value and sets the persistence parameter D to send to each user terminal with the value −1 if the RoT value is greater than a first preset threshold, 1 if the RoT value is less than a second preset threshold, and 0 in all other cases. This technique also reacts to changes in RoT by modifying the parameters of user terminals, but does not allow outage on these same terminals to be prevented.